Several vehicle control systems, which are used to augment the driving capability of a vehicle operator, currently exist. Those control systems include anti-brake-lock system (ABS), traction control system (TCS), and stability controls. Example stability control systems are electronic stability control (ESC) systems or sometimes referred to as yaw stability control (YSC) systems, and roll stability control (RSC) systems. The stability control systems are utilized to maintain controlled and stable vehicle operations for improved vehicle and occupant safety. The stability control systems are often used to maintain control of a vehicle following a desired travel direction, to prevent the vehicle from spinning out, and/or to prevent or mitigate a roll over event.
More specifically, the above YSC systems typically compare the desired direction of a vehicle based upon the steering wheel angle and the path of travel, which is determined from motion sensors located on the vehicle. By regulating the amount of braking at each corner of the vehicle and the traction force of the vehicle, the desired path of travel may be maintained.
Existing stability control systems are designed to correct undesired vehicle motion caused by a tire force disturbance, such as a tire force differential due to a road surface disturbance or due to a mismatch between the driving intention of a driver and a road surface condition. This mismatch usually happens when there is a significant difference between the front and the rear tire lateral forces applied to the vehicle (referred to as the lateral tire force differential), or there is a significant difference between the right and the left tire longitudinal tire forces (referred to as the longitudinal tire force differential), or a combination thereof. Such a tire force differential is called a tire force disturbance (TFD).
The existing YSC systems are effective in controlling the undesired vehicle motions due to the afore-mentioned TFD. The YSC systems activate brakes, which reduces engine torque, or vary the driving torque at individual wheels or axles so as to generate an active tire force differential to counteract the effect of the TFD. That is, the control mechanism and the vehicle disturbance are from the same source: the tire force variations or the tire force differentials.
For example, during a split mu braking event, the tires on the low mu side can be easily locked. Therefore, the tire longitudinal force differential between low mu side and the high mu side is generated. Such a TFD generates a yawing moment disturbance, which causes the vehicle to yaw abnormally towards the high mu side. When the vehicle is equipped with an ABS system, the wheels on the low mu side will be prevented from being locked to maintain a certain level of tire longitudinal forces. In this way, the TFD due to the side-to-side tire longitudinal force differential is reduced and the undesired vehicle yaw motion is reduced.
For another example, when a vehicle is driven at a high speed to negotiate a turn, the vehicle could saturate its front tire cornering forces such that there is a rear-to-front tire lateral force differential. Such a TFD will generate a yaw moment disturbance, which causes the vehicle to steer less than that requested by the driver. This is referred to as an understeer situation. When the existing YSC systems are used, the rear inside wheel is braked briefly to add a longitudinal force to generate a yaw moment to counteract the yaw moment disturbance generated by the TFD due to the rear-to-front tire lateral force differential. Hence the existing YSC systems can help correct an understeer situation due to a TFD.
For a further example, the braking of a vehicle at a turn can cause a large load transfer to the front such that the front tire cornering forces are increased, when the front tire forces before braking are below their saturation levels. Hence a TFD due to the front-to-rear tire cornering force differential is generated, which adds a yaw moment disturbance that causes the vehicle to steer more than that requested by the driver. This is referred to as an oversteer situation. By using the existing YSC systems, the front outside wheel is braked briefly to add a longitudinal force to generate a yaw moment to correct the oversteer. Note that braking in a turn can also cause a vehicle to under-steer when the vehicle forces are close to their saturation levels prior to the braking.
An undesired yaw motion may also be generated by a yaw moment disturbance caused when a vehicle receives a force disturbance other than a tire force disturbance. An example of which is an external force disturbance that is applied to the vehicle body, which is called a body force disturbance (BFD). A BFD may occur when a vehicle hits a fixed object, such as a tree, or when the vehicle is hit by another moving object, such as a moving vehicle or a missile. A BFD may also occur when the vehicle experiences a sudden strong wind gust applied to the vehicle body.
While the magnitude of the tire force disturbance is limited by the driving condition of the road surface, the magnitude of a BFD can be unlimited. For example, the collision of two moving vehicles may generate a body force disturbance with a magnitude that is several factors larger than the total tire forces. In a light collision, the magnitude of the BFD might be very close to the total tire force. The BFD is different from the TFD generated from the tire force differentials. A vehicle experiencing a BFD may have balanced tire forces (i.e., there are no significant tire force differentials among the 4 tires) and may have larger-than-normal vehicle motion such as yaw motion.
A yaw motion may be generated when a vehicle receives a BFD from an external source. Since the tire forces may be well balanced before and after receiving the BFD (i.e., there are no significant tire force differentials among the 4 tires), the existing stability control is ineffective, since it is designed to handle vehicle yaw motion due to a TFD generated from tire differentials. However it is natural to study when the existing stability control actuators can generate a tire force differential to counteract undesired motions due to some class of BFDs.
A BFD is said to be uncontrollable from the tire forces, when both the driving input and the stability control actuation can not generate enough active tire force differentials to counteract the motion of the vehicle caused by the BFD application. This is the case when the BFD is large and it overcomes the tire resistance such that all the tires are well saturated. Due to the saturated tire forces, any manipulation of the tire forces through braking, steering, or changing driving torque cannot generate enough active tire force differentials. Hence the vehicle loses its controllability through both driver steering and the stability control system. This is referred to as a loss-of-control event. One or more embodiments of the present invention control the motion of the vehicle before such a loss-of-control is reached.
Also, current stability, control systems are designed to aid a vehicle driver in pursuing a driver intended action or course. This can result in increased destabilization and/or a collision when a vehicle driver panics, as a result of the impending or actual application of a BFD. During a BFD event, a driver may panic and perform driving tasks that are inappropriate or drastic in an attempt to avoid receiving the external body force disturbance (such as a vehicle-to-vehicle collision or a vehicle-to-missile collision, etc.), which can lead to further undesirable events.
In addition, during a BFD event where a vehicle may be hit from the side, the lateral excursion of the vehicle can result in the vehicle being tripped by a curb or the like or simply by road friction and the large experienced tire forces, due to the large vehicular yaw motion. Additional events can occur due to the abnormal path of the vehicle and the large path deviation.
Thus, there exists a need for an enhanced stability control system that provides improved control action to vehicle yaw motion generated from a controllable body-force disturbance (BFD).